Voters Tuesday in parts of Hampden and Hampshire counties will narrow the field in the special election to replace former state Sen. Mike Knapik.

Knapik left the legislature in early August to take a better paying job at Westfield State University. The Republican lawmaker’s departure thinned GOP ranks in the Senate to just three.

There are two Republicans on the ballot Tuesday vying for the party’s nomination: Donald Humason, a six-term state representative who – for a long time – worked as an aide to Knapik; and Mike Franco, an investigator with the Holyoke veteran services office who’s run – unsuccessfully – in at least five other elections, usually for Governor’s Council.

The Democrats running are Holyoke City Councilor David Bartley and longtime Easthampton Mayor Michael Tautznik.